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Active Record Validations
=========================

This guide teaches you how to validate the state of objects before they go into
the database using Active Record's validations feature.

After reading this guide, you will know:

* How to use the built-in Active Record validation helpers.
* How to create your own custom validation methods.
* How to work with the error messages generated by the validation process.

--------------------------------------------------------------------------------

Validations Overview
--------------------

Here's an example of a very simple validation:

```ruby
class Person < ApplicationRecord
  validates :name, presence: true
end
```

```irb
irb> Person.create(name: "John Doe").valid?
=> true
irb> Person.create(name: nil).valid?
=> false
```

As you can see, our validation lets us know that our `Person` is not valid
without a `name` attribute. The second `Person` will not be persisted to the
database.

Before we dig into more details, let's talk about how validations fit into the
big picture of your application.

### Why Use Validations?

Validations are used to ensure that only valid data is saved into your
database. For example, it may be important to your application to ensure that
every user provides a valid email address and mailing address. Model-level
validations are the best way to ensure that only valid data is saved into your
database. They are database agnostic, cannot be bypassed by end users, and are
convenient to test and maintain. Rails provides built-in helpers for common
needs, and allows you to create your own validation methods as well.

There are several other ways to validate data before it is saved into your
database, including native database constraints, client-side validations and
controller-level validations. Here's a summary of the pros and cons:

* Database constraints and/or stored procedures make the validation mechanisms
  database-dependent and can make testing and maintenance more difficult.
  However, if your database is used by other applications, it may be a good
  idea to use some constraints at the database level. Additionally,
  database-level validations can safely handle some things (such as uniqueness
  in heavily-used tables) that can be difficult to implement otherwise.
* Client-side validations can be useful, but are generally unreliable if used
  alone. If they are implemented using JavaScript, they may be bypassed if
  JavaScript is turned off in the user's browser. However, if combined with
  other techniques, client-side validation can be a convenient way to provide
  users with immediate feedback as they use your site.
* Controller-level validations can be tempting to use, but often become
  unwieldy and difficult to test and maintain. Whenever possible, it's a good
  idea to keep your controllers simple, as it will make your
  application a pleasure to work with in the long run.

Choose these in certain, specific cases. It's the opinion of the Rails team
that model-level validations are the most appropriate in most circumstances.

### When Does Validation Happen?

There are two kinds of Active Record objects: those that correspond to a row
inside your database and those that do not. When you create a fresh object, for
example using the `new` method, that object does not belong to the database
yet. Once you call `save` upon that object it will be saved into the
appropriate database table. Active Record uses the `new_record?` instance
method to determine whether an object is already in the database or not.
Consider the following Active Record class:

```ruby
class Person < ApplicationRecord
end
```

We can see how it works by looking at some `bin/rails console` output:

```irb
irb> p = Person.new(name: "John Doe")
=> #<Person id: nil, name: "John Doe", created_at: nil, updated_at: nil>

irb> p.new_record?
=> true

irb> p.save
=> true

irb> p.new_record?
=> false
```

Creating and saving a new record will send an SQL `INSERT` operation to the
database. Updating an existing record will send an SQL `UPDATE` operation
instead. Validations are typically run before these commands are sent to the
database. If any validations fail, the object will be marked as invalid and
Active Record will not perform the `INSERT` or `UPDATE` operation. This avoids
storing an invalid object in the database. You can choose to have specific
validations run when an object is created, saved, or updated.

CAUTION: There are many ways to change the state of an object in the database.
Some methods will trigger validations, but some will not. This means that it's
possible to save an object in the database in an invalid state if you aren't
careful.

The following methods trigger validations, and will save the object to the
database only if the object is valid:

* `create`
* `create!`
* `save`
* `save!`
* `update`
* `update!`

The bang versions (e.g. `save!`) raise an exception if the record is invalid.
The non-bang versions don't: `save` and `update` return `false`, and
`create` returns the object.

### Skipping Validations

The following methods skip validations, and will save the object to the
database regardless of its validity. They should be used with caution.

* `decrement!`
* `decrement_counter`
* `increment!`
* `increment_counter`
* `insert`
* `insert!`
* `insert_all`
* `insert_all!`
* `toggle!`
* `touch`
* `touch_all`
* `update_all`
* `update_attribute`
* `update_column`
* `update_columns`
* `update_counters`
* `upsert`
* `upsert_all`

Note that `save` also has the ability to skip validations if passed `validate:
false` as an argument. This technique should be used with caution.

* `save(validate: false)`

### `valid?` and `invalid?`

Before saving an Active Record object, Rails runs your validations.
If these validations produce any errors, Rails does not save the object.

You can also run these validations on your own. [`valid?`][] triggers your validations
and returns true if no errors were found in the object, and false otherwise.
As you saw above:

```ruby
class Person < ApplicationRecord
  validates :name, presence: true
end
```

```irb
irb> Person.create(name: "John Doe").valid?
=> true
irb> Person.create(name: nil).valid?
=> false
```

After Active Record has performed validations, any failures can be accessed
through the [`errors`][] instance method, which returns a collection of errors.
By definition, an object is valid if this collection is empty after running
validations.

Note that an object instantiated with `new` will not report errors
even if it's technically invalid, because validations are automatically run
only when the object is saved, such as with the `create` or `save` methods.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true
end
```

```irb
irb> p = Person.new
=> #<Person id: nil, name: nil>
irb> p.errors.size
=> 0

irb> p.valid?
=> false
irb> p.errors.objects.first.full_message
=> "Name can’t be blank"

irb> p = Person.create
=> #<Person id: nil, name: nil>
irb> p.errors.objects.first.full_message
=> "Name can’t be blank"

irb> p.save
=> false

irb> p.save!
ActiveRecord::RecordInvalid: Validation failed: Name can’t be blank

irb> Person.create!
ActiveRecord::RecordInvalid: Validation failed: Name can’t be blank
```

[`invalid?`][] is the inverse of `valid?`. It triggers your validations,
returning true if any errors were found in the object, and false otherwise.

[`errors`]: https://api.rubyonrails.org/classes/ActiveModel/Validations.html#method-i-errors
[`invalid?`]: https://api.rubyonrails.org/classes/ActiveModel/Validations.html#method-i-invalid-3F
[`valid?`]: https://api.rubyonrails.org/classes/ActiveRecord/Validations.html#method-i-valid-3F

### `errors[]`

To verify whether or not a particular attribute of an object is valid, you can
use [`errors[:attribute]`][Errors#squarebrackets]. It returns an array of all
the error messages for `:attribute`. If there are no errors on the specified
attribute, an empty array is returned.

This method is only useful _after_ validations have been run, because it only
inspects the errors collection and does not trigger validations itself. It's
different from the `ActiveRecord::Base#invalid?` method explained above because
it doesn't verify the validity of the object as a whole. It only checks to see
whether there are errors found on an individual attribute of the object.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true
end
```

```irb
irb> Person.new.errors[:name].any?
=> false
irb> Person.create.errors[:name].any?
=> true
```

We'll cover validation errors in greater depth in the [Working with Validation
Errors](#working-with-validation-errors) section.

[Errors#squarebrackets]: https://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-5B-5D

Validation Helpers
------------------

Active Record offers many pre-defined validation helpers that you can use
directly inside your class definitions. These helpers provide common validation
rules. Every time a validation fails, an error is added to the object's `errors`
collection, and this is associated with the attribute being validated.

Each helper accepts an arbitrary number of attribute names, so with a single
line of code you can add the same kind of validation to several attributes.

All of them accept the `:on` and `:message` options, which define when the
validation should be run and what message should be added to the `errors`
collection if it fails, respectively. The `:on` option takes one of the values
`:create` or `:update`. There is a default error message for each one of the
validation helpers. These messages are used when the `:message` option isn't
specified. Let's take a look at each one of the available helpers.

INFO: To see a list of the available default helpers, take a look at
[`ActiveModel::Validations::HelperMethods`][].

[`ActiveModel::Validations::HelperMethods`]: https://api.rubyonrails.org/classes/ActiveModel/Validations/HelperMethods.html

### `acceptance`

This method validates that a checkbox on the user interface was checked when a
form was submitted. This is typically used when the user needs to agree to your
application's terms of service, confirm that some text is read, or any similar
concept.

```ruby
class Person < ApplicationRecord
  validates :terms_of_service, acceptance: true
end
```

This check is performed only if `terms_of_service` is not `nil`.
The default error message for this helper is _"must be accepted"_.
You can also pass in a custom message via the `message` option.

```ruby
class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { message: 'must be abided' }
end
```

It can also receive an `:accept` option, which determines the allowed values
that will be considered as acceptable. It defaults to `['1', true]` and can be
easily changed.

```ruby
class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { accept: 'yes' }
  validates :eula, acceptance: { accept: ['TRUE', 'accepted'] }
end
```

This validation is very specific to web applications and this 'acceptance' does
not need to be recorded anywhere in your database. If you don't have a field for
it, the helper will create a virtual attribute. If the field does exist in your
database, the `accept` option must be set to or include `true` or else the
validation will not run.

### `confirmation`

You should use this helper when you have two text fields that should receive
exactly the same content. For example, you may want to confirm an email address
or a password. This validation creates a virtual attribute whose name is the
name of the field that has to be confirmed with "_confirmation" appended.

```ruby
class Person < ApplicationRecord
  validates :email, confirmation: true
end
```

In your view template you could use something like

```erb
<%= text_field :person, :email %>
<%= text_field :person, :email_confirmation %>
```

NOTE: This check is performed only if `email_confirmation` is not `nil`. To require
confirmation, make sure to add a presence check for the confirmation attribute
(we'll take a look at `presence` [later](#presence) on in this guide):

```ruby
class Person < ApplicationRecord
  validates :email, confirmation: true
  validates :email_confirmation, presence: true
end
```

There is also a `:case_sensitive` option that you can use to define whether the
confirmation constraint will be case sensitive or not. This option defaults to
true.

```ruby
class Person < ApplicationRecord
  validates :email, confirmation: { case_sensitive: false }
end
```

The default error message for this helper is _"doesn't match confirmation"_.
You can also pass in a custom message via the `message` option.

Generally when using this validator, you will want to combine it with the `:if`
option to only validate the "_confirmation" field when the initial field has
changed and **not** every time you save the record. More on [conditional
validations](#conditional-validation) later.

```ruby
class Person < ApplicationRecord
  validates :email, confirmation: true
  validates :email_confirmation, presence: true, if: :email_changed?
end
```

### `comparison`

This check will validate a comparison between any two comparable values.

```ruby
class Promotion < ApplicationRecord
  validates :end_date, comparison: { greater_than: :start_date }
end
```

The default error message for this helper is _"failed comparison"_.
You can also pass in a custom message via the `message` option.

These options are all supported:

* `:greater_than` - Specifies the value must be greater than the supplied
  value. The default error message for this option is _"must be greater than
  %{count}"_.
* `:greater_than_or_equal_to` - Specifies the value must be greater than or
  equal to the supplied value. The default error message for this option is
  _"must be greater than or equal to %{count}"_.
* `:equal_to` - Specifies the value must be equal to the supplied value. The
  default error message for this option is _"must be equal to %{count}"_.
* `:less_than` - Specifies the value must be less than the supplied value. The
  default error message for this option is _"must be less than %{count}"_.
* `:less_than_or_equal_to` - Specifies the value must be less than or equal to
  the supplied value. The default error message for this option is _"must be
  less than or equal to %{count}"_.
* `:other_than` - Specifies the value must be other than the supplied value.
  The default error message for this option is _"must be other than %{count}"_.

NOTE: The validator requires a compare option be supplied. Each option accepts a
value, proc, or symbol. Any class that includes Comparable can be compared.

### `format`

This helper validates the attributes' values by testing whether they match a
given regular expression, which is specified using the `:with` option.

```ruby
class Product < ApplicationRecord
  validates :legacy_code, format: { with: /\A[a-zA-Z]+\z/,
    message: "only allows letters" }
end
```

Inversely, by using the `:without` option instead you can require that the
specified attribute does _not_ match the regular expression.

In either case, the provided `:with` or `:without` option must be a regular
expression or a proc or lambda that return one.

The default error message is _"is invalid"_.

WARNING. use `\A` and `\z` to match the start and end of the string, `^` and `$`
match the start/end of a line. Due to frequent misuse of `^` and `$`, you need
to pass the `multiline: true` option in case you use any of these two anchors in
the provided regular expression. In most cases, you should be using `\A` and
`\z`.

### `inclusion`

This helper validates that the attributes' values are included in a given set.
In fact, this set can be any enumerable object.

```ruby
class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }
end
```

The `inclusion` helper has an option `:in` that receives the set of values that
will be accepted. The `:in` option has an alias called `:within` that you can
use for the same purpose, if you'd like to. The previous example uses the
`:message` option to show how you can include the attribute's value. For full
options please see the [message documentation](#message).

The default error message for this helper is _"is not included in the list"_.

### `exclusion`

The opposite of `inclusion` is... `exclusion`!

This helper validates that the attributes' values are not included in a given
set. In fact, this set can be any enumerable object.

```ruby
class Account < ApplicationRecord
  validates :subdomain, exclusion: { in: %w(www us ca jp),
    message: "%{value} is reserved." }
end
```

The `exclusion` helper has an option `:in` that receives the set of values that
will not be accepted for the validated attributes. The `:in` option has an
alias called `:within` that you can use for the same purpose, if you'd like to.
This example uses the `:message` option to show how you can include the
attribute's value. For full options to the message argument please see the
[message documentation](#message).

The default error message is _"is reserved"_.

Alternatively to a traditional enumerable (like an Array), you can supply a
proc, lambda, or symbol which returns an enumerable. If the enumerable is a
numerical, time, or datetime range the test is performed with `Range#cover?`,
otherwise with `include?`. When using a proc or lambda the instance under
validation is passed as an argument.

### `length`

This helper validates the length of the attributes' values. It provides a
variety of options, so you can specify length constraints in different ways:

```ruby
class Person < ApplicationRecord
  validates :name, length: { minimum: 2 }
  validates :bio, length: { maximum: 500 }
  validates :password, length: { in: 6..20 }
  validates :registration_number, length: { is: 6 }
end
```

The possible length constraint options are:

* `:minimum` - The attribute cannot have less than the specified length.
* `:maximum` - The attribute cannot have more than the specified length.
* `:in` (or `:within`) - The attribute length must be included in a given
  interval. The value for this option must be a range.
* `:is` - The attribute length must be equal to the given value.

The default error messages depend on the type of length validation being
performed. You can customize these messages using the `:wrong_length`,
`:too_long`, and `:too_short` options and `%{count}` as a placeholder for the
number corresponding to the length constraint being used. You can still use the
`:message` option to specify an error message.

```ruby
class Person < ApplicationRecord
  validates :bio, length: { maximum: 1000,
    too_long: "%{count} characters is the maximum allowed" }
end
```

Note that the default error messages are plural (e.g., "is too short (minimum
is %{count} characters)"). For this reason, when `:minimum` is 1 you should
provide a custom message or use `presence: true` instead. When
`:in` or `:within` have a lower limit of 1, you should either provide a
custom message or call `presence` prior to `length`.

NOTE: Only one constraint option can be used at a time apart from the `:minimum`
and `:maximum` options which can be combined together.

### `numericality`

This helper validates that your attributes have only numeric values. By
default, it will match an optional sign followed by an integer or floating
point number.

To specify that only integer numbers are allowed, set `:only_integer` to true.
Then it will use the following regular expression to validate the attribute's
value.

```ruby
/\A[+-]?\d+\z/
```

Otherwise, it will try to convert the value to a number using `Float`. `Float`s
are casted to `BigDecimal` using the column's precision value or a maximum of 15
digits.

```ruby
class Player < ApplicationRecord
  validates :points, numericality: true
  validates :games_played, numericality: { only_integer: true }
end
```

The default error message for `:only_integer` is _"must be an integer"_.

Besides `:only_integer`, this helper also accepts the `:only_numeric` option
which specifies the value must be an instance of `Numeric` and attempts to parse
the value if it is a `String`.

NOTE: By default, `numericality` doesn't allow `nil` values. You can use
`allow_nil: true` option to permit it. Notice that for `Integer` and `Float`
columns empty strings are converted to `nil`.

The default error message when no options are specified is _"is not a number"_.

There are also many options that can be used to add constraints to acceptable
values:

* `:greater_than` - Specifies the value must be greater than the supplied
  value. The default error message for this option is _"must be greater than
  %{count}"_.
* `:greater_than_or_equal_to` - Specifies the value must be greater than or
  equal to the supplied value. The default error message for this option is
  _"must be greater than or equal to %{count}"_.
* `:equal_to` - Specifies the value must be equal to the supplied value. The
  default error message for this option is _"must be equal to %{count}"_.
* `:less_than` - Specifies the value must be less than the supplied value. The
  default error message for this option is _"must be less than %{count}"_.
* `:less_than_or_equal_to` - Specifies the value must be less than or equal to
  the supplied value. The default error message for this option is _"must be
  less than or equal to %{count}"_.
* `:other_than` - Specifies the value must be other than the supplied value.
  The default error message for this option is _"must be other than %{count}"_.
* `:in` - Specifies the value must be in the supplied range.
  The default error message for this option is _"must be in %{count}"_.
* `:odd` - Specifies the value must be an odd number. The default error message
  for this option is _"must be odd"_.
* `:even` - Specifies the value must be an even number. The default error
  message for this option is _"must be even"_.

### `presence`

This helper validates that the specified attributes are not empty. It uses the
[`Object#blank?`][] method to check if the value is either `nil` or a blank
string, that is, a string that is either empty or consists of whitespace.

```ruby
class Person < ApplicationRecord
  validates :name, :login, :email, presence: true
end
```

If you want to be sure that an association is present, you'll need to test
whether the associated object itself is present, and not the foreign key used
to map the association. This way, it is not only checked that the foreign key
is not empty but also that the referenced object exists.

```ruby
class Supplier < ApplicationRecord
  has_one :account
  validates :account, presence: true
end
```

In order to validate associated records whose presence is required, you must
specify the `:inverse_of` option for the association:

```ruby
class Order < ApplicationRecord
  has_many :line_items, inverse_of: :order
end
```

NOTE: If you want to ensure that the association it is both present and valid,
you also need to use `validates_associated`. More on that
[below](#validates-associated)

If you validate the presence of an object associated via a `has_one` or
`has_many` relationship, it will check that the object is neither `blank?` nor
`marked_for_destruction?`.

Since `false.blank?` is true, if you want to validate the presence of a boolean
field you should use one of the following validations:

```ruby
# Value _must_ be true or false
validates :boolean_field_name, inclusion: [true, false]
# Value _must not_ be nil, aka true or false
validates :boolean_field_name, exclusion: [nil]
```

By using one of these validations, you will ensure the value will NOT be `nil`
which would result in a `NULL` value in most cases.

The default error message is _"can’t be blank"_.

[`Object#blank?`]: https://api.rubyonrails.org/classes/Object.html#method-i-blank-3F

### `absence`

This helper validates that the specified attributes are absent. It uses the
[`Object#present?`][] method to check if the value is not either nil or a blank
string, that is, a string that is either empty or consists of whitespace.

```ruby
class Person < ApplicationRecord
  validates :name, :login, :email, absence: true
end
```

If you want to be sure that an association is absent, you'll need to test
whether the associated object itself is absent, and not the foreign key used
to map the association.

```ruby
class LineItem < ApplicationRecord
  belongs_to :order
  validates :order, absence: true
end
```

In order to validate associated records whose absence is required, you must
specify the `:inverse_of` option for the association:

```ruby
class Order < ApplicationRecord
  has_many :line_items, inverse_of: :order
end
```

NOTE: If you want to ensure that the association it is both present and valid,
you also need to use `validates_associated`. More on that
[below](#validates-associated)

If you validate the absence of an object associated via a `has_one` or
`has_many` relationship, it will check that the object is neither `present?` nor
`marked_for_destruction?`.

Since `false.present?` is false, if you want to validate the absence of a boolean
field you should use `validates :field_name, exclusion: { in: [true, false] }`.

The default error message is _"must be blank"_.

[`Object#present?`]: https://api.rubyonrails.org/classes/Object.html#method-i-present-3F

### `uniqueness`

This helper validates that the attribute's value is unique right before the
object gets saved.

```ruby
class Account < ApplicationRecord
  validates :email, uniqueness: true
end
```

The validation happens by performing an SQL query into the model's table,
searching for an existing record with the same value in that attribute.

There is a `:scope` option that you can use to specify one or more attributes that
are used to limit the uniqueness check:

```ruby
class Holiday < ApplicationRecord
  validates :name, uniqueness: { scope: :year,
    message: "should happen once per year" }
end
```

WARNING. This validation does not create a uniqueness constraint in the
database, so it may happen that two different database connections create two
records with the same value for a column that you intend to be unique. To avoid
that, you must create a unique index on that column in your database.

In order to add a uniqueness database constraint on your database, use the
[`add_index`][] statement in a migration and include the `unique: true` option.

Should you wish to create a database constraint to prevent possible violations
of a uniqueness validation using the `:scope` option, you must create a unique
index on both columns in your database. See [the MySQL manual][] for more
details about multiple column indexes or [the PostgreSQL manual][] for examples
of unique constraints that refer to a group of columns.

There is also a `:case_sensitive` option that you can use to define whether the
uniqueness constraint will be case sensitive, case insensitive, or respects
default database collation. This option defaults to respects default database
collation.

```ruby
class Person < ApplicationRecord
  validates :name, uniqueness: { case_sensitive: false }
end
```

WARNING. Note that some databases are configured to perform case-insensitive
searches anyway.

There is a `:conditions` option that you can specify additional conditions as a
`WHERE` SQL fragment to limit the uniqueness constraint lookup (e.g.
`conditions: -> { where(status: 'active') }`).

The default error message is _"has already been taken"_.

See [`validates_uniqueness_of`][] for more information.

[`validates_uniqueness_of`]: https://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of
[`add_index`]: https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index
[the MySQL manual]: https://dev.mysql.com/doc/refman/en/multiple-column-indexes.html
[the PostgreSQL manual]: https://www.postgresql.org/docs/current/static/ddl-constraints.html

### `validates_associated`

You should use this helper when your model has associations that always need to
be validated. Every time you try to save your object, `valid?` will be called
on each one of the associated objects.

```ruby
class Library < ApplicationRecord
  has_many :books
  validates_associated :books
end
```

This validation will work with all of the association types.

CAUTION: Don't use `validates_associated` on both ends of your associations.
They would call each other in an infinite loop.

The default error message for [`validates_associated`][] is _"is invalid"_. Note
that each associated object will contain its own `errors` collection; errors do
not bubble up to the calling model.

NOTE: [`validates_associated`][] can only be used with ActiveRecord objects,
everything up until now can also be used on any object which includes
[`ActiveModel::Validations`][].

[`validates_associated`]: https://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_associated

### `validates_each`

This helper validates attributes against a block. It doesn't have a predefined
validation function. You should create one using a block, and every attribute
passed to [`validates_each`][] will be tested against it.

In the following example, we will reject names and surnames that begin with
lower case.

```ruby
class Person < ApplicationRecord
  validates_each :name, :surname do |record, attr, value|
    record.errors.add(attr, 'must start with upper case') if /\A[[:lower:]]/.match?(value)
  end
end
```

The block receives the record, the attribute's name, and the attribute's value.

You can do anything you like to check for valid data within the block. If your
validation fails, you should add an error to the model, therefore making it
invalid.

[`validates_each`]: https://api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html#method-i-validates_each

### `validates_with`

This helper passes the record to a separate class for validation.

```ruby
class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if record.first_name == "Evil"
      record.errors.add :base, "This person is evil"
    end
  end
end

class Person < ApplicationRecord
  validates_with GoodnessValidator
end
```

There is no default error message for `validates_with`. You must manually add
errors to the record's errors collection in the validator class.

NOTE: Errors added to `record.errors[:base]` relate to the state of the record
as a whole.

To implement the validate method, you must accept a `record` parameter in the
method definition, which is the record to be validated.

If you want to add an error on a specific attribute, pass it as the first
argument, such as `record.errors.add(:first_name, "please choose another
name")`. We will cover [validation errors][] in greater detail later.

```ruby
def validate(record)
  if record.some_field != "acceptable"
    record.errors.add :some_field, "this field is unacceptable"
  end
end
```

The [`validates_with`][] helper takes a class, or a list of classes to use for
validation.

```ruby
class Person < ApplicationRecord
  validates_with MyValidator, MyOtherValidator, on: :create
end
```

Like all other validations, `validates_with` takes the `:if`, `:unless` and
`:on` options. If you pass any other options, it will send those options to the
validator class as `options`:

```ruby
class GoodnessValidator < ActiveModel::Validator
  def validate(record)
    if options[:fields].any? { |field| record.send(field) == "Evil" }
      record.errors.add :base, "This person is evil"
    end
  end
end

class Person < ApplicationRecord
  validates_with GoodnessValidator, fields: [:first_name, :last_name]
end
```

Note that the validator will be initialized *only once* for the whole application
life cycle, and not on each validation run, so be careful about using instance
variables inside it.

If your validator is complex enough that you want instance variables, you can
easily use a plain old Ruby object instead:

```ruby
class Person < ApplicationRecord
  validate do |person|
    GoodnessValidator.new(person).validate
  end
end

class GoodnessValidator
  def initialize(person)
    @person = person
  end

  def validate
    if some_complex_condition_involving_ivars_and_private_methods?
      @person.errors.add :base, "This person is evil"
    end
  end

  # ...
end
```

We will cover [custom validations](#performing-custom-validations) more later.

[validation errors](#working-with-validation-errors)
[`validates_with`]: https://api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html#method-i-validates_with

Common Validation Options
-------------------------

There are several common options supported by the validators we just went over,
let's go over some of them now!

NOTE: Not all of these options are supported by every validator, please refer to
the API documentation for [`ActiveModel::Validations`][].

By using any of the validation methods we just mentioned, there also is a list
of common options shared along with validators. We will cover these now!

* [`:allow_nil`](#allow-nil): Skip validation if the attribute is `nil`.
* [`:allow_blank`](#allow-blank): Skip validation if the attribute is blank.
* [`:message`](#message): Specify a custom error message.
* [`:on`](#on): Specify the contexts where this validation is active.
* [`:strict`](#strict-validations): Raise an exception when the validation
  fails.
* [`:if` and `:unless`](#conditional-validation): Specify when the validation
  should or should not occur.

[`ActiveModel::Validations`]: https://api.rubyonrails.org/classes/ActiveModel/Validations.html

### `:allow_nil`

The `:allow_nil` option skips the validation when the value being validated is
`nil`.

```ruby
class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }, allow_nil: true
end
```

```irb
irb> Coffee.create(size: nil).valid?
=> true
irb> Coffee.create(size: "mega").valid?
=> false
```

For full options to the message argument please see the
[message documentation](#message).

### `:allow_blank`

The `:allow_blank` option is similar to the `:allow_nil` option. This option
will let validation pass if the attribute's value is `blank?`, like `nil` or an
empty string for example.

```ruby
class Topic < ApplicationRecord
  validates :title, length: { is: 5 }, allow_blank: true
end
```

```irb
irb> Topic.create(title: "").valid?
=> true
irb> Topic.create(title: nil).valid?
=> true
```

### `:message`

As you've already seen, the `:message` option lets you specify the message that
will be added to the `errors` collection when validation fails. When this
option is not used, Active Record will use the respective default error message
for each validation helper.

The `:message` option accepts either a `String` or `Proc` as its value.

A `String` `:message` value can optionally contain any/all of `%{value}`,
`%{attribute}`, and `%{model}` which will be dynamically replaced when
validation fails. This replacement is done using the i18n gem, and the
placeholders must match exactly, no spaces are allowed.

```ruby
class Person < ApplicationRecord
  # Hard-coded message
  validates :name, presence: { message: "must be given please" }

  # Message with dynamic attribute value. %{value} will be replaced
  # with the actual value of the attribute. %{attribute} and %{model}
  # are also available.
  validates :age, numericality: { message: "%{value} seems wrong" }
end
```

A `Proc` `:message` value is given two arguments: the object being validated, and
a hash with `:model`, `:attribute`, and `:value` key-value pairs.

```ruby
class Person < ApplicationRecord
  validates :username,
    uniqueness: {
      # object = person object being validated
      # data = { model: "Person", attribute: "Username", value: <username> }
      message: ->(object, data) do
        "Hey #{object.name}, #{data[:value]} is already taken."
      end
    }
end
```

### `:on`

The `:on` option lets you specify when the validation should happen. The
default behavior for all the built-in validation helpers is to be run on save
(both when you're creating a new record and when you're updating it). If you
want to change it, you can use `on: :create` to run the validation only when a
new record is created or `on: :update` to run the validation only when a record
is updated.

```ruby
class Person < ApplicationRecord
  # it will be possible to update email with a duplicated value
  validates :email, uniqueness: true, on: :create

  # it will be possible to create the record with a non-numerical age
  validates :age, numericality: true, on: :update

  # the default (validates on both create and update)
  validates :name, presence: true
end
```

You can also use `on:` to define custom contexts. Custom contexts need to be
triggered explicitly by passing the name of the context to `valid?`,
`invalid?`, or `save`.

```ruby
class Person < ApplicationRecord
  validates :email, uniqueness: true, on: :account_setup
  validates :age, numericality: true, on: :account_setup
end
```

```irb
irb> person = Person.new(age: 'thirty-three')
irb> person.valid?
=> true
irb> person.valid?(:account_setup)
=> false
irb> person.errors.messages
=> {:email=>["has already been taken"], :age=>["is not a number"]}
```

`person.valid?(:account_setup)` executes both the validations without saving
the model. `person.save(context: :account_setup)` validates `person` in the
`account_setup` context before saving.

Passing an array of symbols is also acceptable.

```ruby
class Book
  include ActiveModel::Validations

  validates :title, presence: true, on: [:update, :ensure_title]
end
```

```irb
irb> book = Book.new(title: nil)
irb> book.valid?
=> true
irb> book.valid?(:ensure_title)
=> false
irb> book.errors.messages
=> {:title=>["can’t be blank"]}
```

When triggered by an explicit context, validations are run for that context,
as well as any validations _without_ a context.

```ruby
class Person < ApplicationRecord
  validates :email, uniqueness: true, on: :account_setup
  validates :age, numericality: true, on: :account_setup
  validates :name, presence: true
end
```

```irb
irb> person = Person.new
irb> person.valid?(:account_setup)
=> false
irb> person.errors.messages
=> {:email=>["has already been taken"], :age=>["is not a number"], :name=>["can’t be blank"]}
```

We will cover more use-cases for `on:` in the [callbacks guide](active_record_callbacks.html).

Strict Validations
------------------

You can also specify validations to be strict and raise
`ActiveModel::StrictValidationFailed` when the object is invalid.

```ruby
class Person < ApplicationRecord
  validates :name, presence: { strict: true }
end
```

```irb
irb> Person.new.valid?
ActiveModel::StrictValidationFailed: Name can’t be blank
```

There is also the ability to pass a custom exception to the `:strict` option.

```ruby
class Person < ApplicationRecord
  validates :token, presence: true, uniqueness: true, strict: TokenGenerationException
end
```

```irb
irb> Person.new.valid?
TokenGenerationException: Token can’t be blank
```

Conditional Validation
----------------------

Sometimes it will make sense to validate an object only when a given predicate
is satisfied. You can do that by using the `:if` and `:unless` options, which
can take a symbol, a `Proc` or an `Array`. You may use the `:if` option when you
want to specify when the validation **should** happen. Alternatively, if you
want to specify when the validation **should not** happen, then you may use the
`:unless` option.

### Using a Symbol with `:if` and `:unless`

You can associate the `:if` and `:unless` options with a symbol corresponding
to the name of a method that will get called right before validation happens.
This is the most commonly used option.

```ruby
class Order < ApplicationRecord
  validates :card_number, presence: true, if: :paid_with_card?

  def paid_with_card?
    payment_type == "card"
  end
end
```

### Using a Proc with `:if` and `:unless`

It is possible to associate `:if` and `:unless` with a `Proc` object
which will be called. Using a `Proc` object gives you the ability to write an
inline condition instead of a separate method. This option is best suited for
one-liners.

```ruby
class Account < ApplicationRecord
  validates :password, confirmation: true,
    unless: Proc.new { |a| a.password.blank? }
end
```

As `lambda` is a type of `Proc`, it can also be used to write inline conditions
taking advantage of the shortened syntax.

```ruby
validates :password, confirmation: true, unless: -> { password.blank? }
```

### Grouping Conditional Validations

Sometimes it is useful to have multiple validations use one condition. It can
be easily achieved using [`with_options`][].

```ruby
class User < ApplicationRecord
  with_options if: :is_admin? do |admin|
    admin.validates :password, length: { minimum: 10 }
    admin.validates :email, presence: true
  end
end
```

All validations inside of the `with_options` block will have automatically
passed the condition `if: :is_admin?`

[`with_options`]: https://api.rubyonrails.org/classes/Object.html#method-i-with_options

### Combining Validation Conditions

On the other hand, when multiple conditions define whether or not a validation
should happen, an `Array` can be used. Moreover, you can apply both `:if` and
`:unless` to the same validation.

```ruby
class Computer < ApplicationRecord
  validates :mouse, presence: true,
                    if: [Proc.new { |c| c.market.retail? }, :desktop?],
                    unless: Proc.new { |c| c.trackpad.present? }
end
```

The validation only runs when all the `:if` conditions and none of the `:unless`
conditions are evaluated to `true`.

Performing Custom Validations
-----------------------------

When the built-in validation helpers are not enough for your needs, you can
write your own validators or validation methods as you prefer.

### Custom Validators

Custom validators are classes that inherit from [`ActiveModel::Validator`][].
These classes must implement the `validate` method which takes a record as an
argument and performs the validation on it. The custom validator is called using
the `validates_with` method.

```ruby
class MyValidator < ActiveModel::Validator
  def validate(record)
    unless record.name.start_with? 'X'
      record.errors.add :name, "Provide a name starting with X, please!"
    end
  end
end

class Person < ApplicationRecord
  validates_with MyValidator
end
```

The easiest way to add custom validators for validating individual attributes
is with the convenient [`ActiveModel::EachValidator`][]. In this case, the custom
validator class must implement a `validate_each` method which takes three
arguments: record, attribute, and value. These correspond to the instance, the
attribute to be validated, and the value of the attribute in the passed
instance.

```ruby
class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless URI::MailTo::EMAIL_REGEXP.match?(value)
      record.errors.add attribute, (options[:message] || "is not an email")
    end
  end
end

class Person < ApplicationRecord
  validates :email, presence: true, email: true
end
```

As shown in the example, you can also combine standard validations with your
own custom validators.

[`ActiveModel::EachValidator`]: https://api.rubyonrails.org/classes/ActiveModel/EachValidator.html
[`ActiveModel::Validator`]: https://api.rubyonrails.org/classes/ActiveModel/Validator.html

### Custom Methods

You can also create methods that verify the state of your models and add errors
to the `errors` collection when they are invalid. You must then register these
methods by using the [`validate`][] class method, passing in the symbols for the
validation methods' names.

You can pass more than one symbol for each class method and the respective
validations will be run in the same order as they were registered.

The `valid?` method will verify that the `errors` collection is empty,
so your custom validation methods should add errors to it when you
wish validation to fail:

```ruby
class Invoice < ApplicationRecord
  validate :expiration_date_cannot_be_in_the_past,
    :discount_cannot_be_greater_than_total_value

  def expiration_date_cannot_be_in_the_past
    if expiration_date.present? && expiration_date < Date.today
      errors.add(:expiration_date, "can't be in the past")
    end
  end

  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors.add(:discount, "can't be greater than total value")
    end
  end
end
```

By default, such validations will run every time you call `valid?` or save the
object. But it is also possible to control when to run these custom validations
by giving an `:on` option to the `validate` method, with either: `:create` or
`:update`.

```ruby
class Invoice < ApplicationRecord
  validate :active_customer, on: :create

  def active_customer
    errors.add(:customer_id, "is not active") unless customer.active?
  end
end
```

See the section above for more details about [`:on`](#on).

### Listing Validators

If you want to find out all of the validators for a given objects, look no
further than `validators`.

For example, if we have the following model using a custom validator and a
built-in validator:

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, on: :create
  validates :email, format: URI::MailTo::EMAIL_REGEXP
  validates_with MyOtherValidator, strict: true
end
```

We can now use `validators` on the "Person" model to list all validators, or
even check a specific field using `validators_on`.

```irb
irb> Person.validators
#=> [#<ActiveRecord::Validations::PresenceValidator:0x10b2f2158
      @attributes=[:name], @options={:on=>:create}>,
     #<MyOtherValidatorValidator:0x10b2f17d0
      @attributes=[:name], @options={:strict=>true}>,
     #<ActiveModel::Validations::FormatValidator:0x10b2f0f10
      @attributes=[:email],
      @options={:with=>URI::MailTo::EMAIL_REGEXP}>]
     #<MyOtherValidator:0x10b2f0948 @options={:strict=>true}>]

irb> Person.validators_on(:name)
#=> [#<ActiveModel::Validations::PresenceValidator:0x10b2f2158
      @attributes=[:name], @options={on: :create}>]
```

[`validate`]: https://api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html#method-i-validate

Working with Validation Errors
------------------------------

The [`valid?`][] and [`invalid?`][] methods only provide a summary status on
validity. However you can dig deeper into each individual error by using various
methods from the [`errors`][] collection.

The following is a list of the most commonly used methods. Please refer to the
[`ActiveModel::Errors`][] documentation for a list of all the available methods.

[`ActiveModel::Errors`]: https://api.rubyonrails.org/classes/ActiveModel/Errors.html

### `errors`

The gateway through which you can drill down into various details of each error.

This returns an instance of the class `ActiveModel::Errors` containing all
errors, each error is represented by an [`ActiveModel::Error`][] object.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end
```

```irb
irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.full_messages
=> ["Name can’t be blank", "Name is too short (minimum is 3 characters)"]

irb> person = Person.new(name: "John Doe")
irb> person.valid?
=> true
irb> person.errors.full_messages
=> []

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.first.details
=> {:error=>:too_short, :count=>3}
```

[`ActiveModel::Error`]: https://api.rubyonrails.org/classes/ActiveModel/Error.html

### `errors[]`

[`errors[]`][Errors#squarebrackets] is used when you want to check the error
messages for a specific attribute. It returns an array of strings with all error
messages for the given attribute, each string with one error message. If there
are no errors related to the attribute, it returns an empty array.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end
```

```irb
irb> person = Person.new(name: "John Doe")
irb> person.valid?
=> true
irb> person.errors[:name]
=> []

irb> person = Person.new(name: "JD")
irb> person.valid?
=> false
irb> person.errors[:name]
=> ["is too short (minimum is 3 characters)"]

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors[:name]
=> ["can’t be blank", "is too short (minimum is 3 characters)"]
```

### `errors.where` and Error Object

Sometimes we may need more information about each error besides its message.
Each error is encapsulated as an `ActiveModel::Error` object, and the
[`where`][] method is the most common way of access.

`where` returns an array of error objects filtered by various degrees of
conditions.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end
```

We can filter for just the `attribute` by passing it as the first parameter to
`errors.where(:attr)`. The second parameter is used for filtering the `type` of
error we want by calling `errors.where(:attr, :type)`.

```irb
irb> person = Person.new
irb> person.valid?
=> false

irb> person.errors.where(:name)
=> [ ... ] # all errors for :name attribute

irb> person.errors.where(:name, :too_short)
=> [ ... ] # :too_short errors for :name attribute
```

Lastly, we can filter by any `options` that may exist on the given type of error
object.

```irb
irb> person = Person.new
irb> person.valid?
=> false

irb> person.errors.where(:name, :too_short, minimum: 3)
=> [ ... ] # all name errors being too short and minimum is 2
```

You can read various information from these error objects:

```irb
irb> error = person.errors.where(:name).last

irb> error.attribute
=> :name
irb> error.type
=> :too_short
irb> error.options[:count]
=> 3
```

You can also generate the error message:

```irb
irb> error.message
=> "is too short (minimum is 3 characters)"
irb> error.full_message
=> "Name is too short (minimum is 3 characters)"
```

The [`full_message`][] method generates a more user-friendly message, with the
capitalized attribute name prepended. (To customize the format that
`full_message` uses, see the [I18n guide](i18n.html#active-model-methods).)

[`full_message`]: https://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-full_message
[`where`]: https://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-where

### `errors.add`

The [`add`][] method creates the error object by taking the `attribute`, the
error `type` and additional options hash. This is useful when writing your own
validator, as it lets you define very specific error situations.

```ruby
class Person < ApplicationRecord
  validate do |person|
    errors.add :name, :too_plain, message: "is not cool enough"
  end
end
```

```irb
irb> person = Person.create
irb> person.errors.where(:name).first.type
=> :too_plain
irb> person.errors.where(:name).first.full_message
=> "Name is not cool enough"
```

[`add`]: https://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-add

### `errors[:base]`

You can add errors that are related to the object's state as a whole, instead of
being related to a specific attribute. To do this you must use `:base` as the
attribute when adding a new error.

```ruby
class Person < ApplicationRecord
  validate do |person|
    errors.add :base, :invalid, message: "This person is invalid because ..."
  end
end
```

```irb
irb> person = Person.create
irb> person.errors.where(:base).first.full_message
=> "This person is invalid because ..."
```

### `errors.size`

The `size` method returns the total number of errors for the object.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end
```

```irb
irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.size
=> 2

irb> person = Person.new(name: "Andrea", email: "andrea@example.com")
irb> person.valid?
=> true
irb> person.errors.size
=> 0
```

### `errors.clear`

The `clear` method is used when you intentionally want to clear the `errors`
collection. Of course, calling `errors.clear` upon an invalid object won't
actually make it valid: the `errors` collection will now be empty, but the next
time you call `valid?` or any method that tries to save this object to the
database, the validations will run again. If any of the validations fail, the
`errors` collection will be filled again.

```ruby
class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end
```

```irb
irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.empty?
=> false

irb> person.errors.clear
irb> person.errors.empty?
=> true

irb> person.save
=> false

irb> person.errors.empty?
=> false
```

Displaying Validation Errors in Views
-------------------------------------

Once you've created a model and added validations, if that model is created via
a web form, you probably want to display an error message when one of the
validations fails.

Because every application handles this kind of thing differently, Rails does
not include any view helpers to help you generate these messages directly.
However, due to the rich number of methods Rails gives you to interact with
validations in general, you can build your own. In addition, when
generating a scaffold, Rails will put some ERB into the `_form.html.erb` that
it generates that displays the full list of errors on that model.

Assuming we have a model that's been saved in an instance variable named
`@article`, it looks like this:

```html+erb
<% if @article.errors.any? %>
  <div id="error_explanation">
    <h2><%= pluralize(@article.errors.count, "error") %> prohibited this article from being saved:</h2>

    <ul>
      <% @article.errors.each do |error| %>
        <li><%= error.full_message %></li>
      <% end %>
    </ul>
  </div>
<% end %>
```

Furthermore, if you use the Rails form helpers to generate your forms, when
a validation error occurs on a field, it will generate an extra `<div>` around
the entry.

```html
<div class="field_with_errors">
  <input id="article_title" name="article[title]" size="30" type="text" value="">
</div>
```

You can then style this div however you'd like. The default scaffold that
Rails generates, for example, adds this CSS rule:

```css
.field_with_errors {
  padding: 2px;
  background-color: red;
  display: table;
}
```

This means that any field with an error ends up with a 2 pixel red border.
